This invention relates to a nuclear power plant, the reactor being of the pressurized-water coolant type and its coolant being circulated through a steam generator having a feed water inlet and in which the feed water generates steam which is used to power a steam engine, such as a turbine, having a low pressure stage and a high pressure stage, the mechanical rotary power produced by the turbine driving a generator producing electric power, for example.
The steam generator is in the form of a vertical cylindrical casing containing a horizontal circular tube sheet in which the ends of the vertical legs of an inverted U-shaped nest of heat exchanging tubes are mounted, the casing below this tube sheet being constructed to form a coolant inlet manifold for the inlet end of the tube nest, and a coolant outlet manifold for the outlet end of the next. The manifolds are in circuit with the coolant circuit of the reactor, the coolant 2 passing through the tube nest. The casing extends above the tube sheet and its upper portion is provided with a steam outlet, a feed water inlet supplying feed water to the casing above the tube sheet and which vaporizes and generates steam. The feed water must be pumped into the casing against the steam pressure by one or more feed water pumps and the feed water is introduced to the water already in the casing, through a feed water reheater inside of the casing and receiving heat from the outlet leg of the tube nest. Above the tube nest the steam generator's casing contains at least one water separator for removing water from the generated steam before leaving the steam generator, and possibly a coarse water separator followed by a fine water separator.
The steam from the steam generator goes to the high pressure stage of a turbine, the exhaust from there then going through a reheater and to a low pressure stage of the turbine, the exhaust of which is handled in a normal manner.
The above steam reheater is a heat exchanger of the counter flow type requiring a supply of hot fluid, and this supply has heretofore been steam taken from the output of the steam generator, thus placing on the latter an extra load above that required to power the turbine. When the turbine load changed unstable operation was possible, condensate shock or backup of the steam condensate discharged from the reheater necessitating the reheater being designed as a heat exchanger of larger size when might otherwise have been necessary. Also, the additional load on the steam generator required the latter to be operated at higher capacity, placing undesirable stress on the steam lines and fittings and on the water separators within the steam generator. Because more steam was required than was necessary to power the turbine, more feed water had to be constantly introduced to the steam generator, requiring a larger capacity feed water pump than what otherwise would have been necessary. Using steam as the heating medium for the reheater, the latter had to be provided a condensate cooler for the exhausting steam, a condensate decompressor and other equipment.
Because of the need for additional steam required by the reheater, a larger amount of water was required to be boiled off in the steam generator, leaving undesirable deposits of salts and the like unavoidably included by the feed water. To constantly circulate the water in the steam generator, the latter had to be provided with an annular descent space surrounding the tube nest with its lower end spaced above the tube sheet to provide for thermal circulation, requiring the generator casing to be larger and therefore more expensive.
During start-up and shut-down of the nuclear reactor, a complicated system of multiple emergency feed water pump equipment, emergency feed water lines and deionate supply tanks, were required to remove the residual heat via the steam generator functioning as a heat exchanger.